1. Technical Field
The present invention relates to a photomask having a halftone region, a semiconductor device manufactured using the photomask, and a method for manufacturing the semiconductor device.
2. Related Art
A semiconductor device is manufactured by the following procedure: for example, a semiconductor layer is formed on a substrate and a resist layer is formed on the semiconductor layer and then processed using a halftone photomask so as to have steps as described in JP-A-2006-54424.
In particular, the photomask has a light-blocking region completely blocking light, a transflective region reducing the intensity of light, and a light-transmissive region transmitting light. The resist layer is exposed to light through the photomask, whereby the following portions are formed in the resist layer: a thick portion corresponding to a channel region and thin portions each corresponding to a source region or a drain region.
The semiconductor layer is patterned using the stepped resist layer as a mask, whereby the source and drain regions are formed in the semiconductor layer so as to have a small thickness. The source and drain regions are doped with an impurity by making use of the difference in thickness between the thick and thin portions of the resist layer.
When the resist layer is processed so as to be stepped, light leaking from the light-transmissive region or reflected by a surrounding portion enters the transflective region, which has a small thickness. Hence, a thin boundary zone of the resist layer that corresponds to the boundary between the transflective region and the light-transmissive region is exposed to light with an intensity greater than that of regular light. In other words, it is difficult to control the intensity of light applied to the boundary zone. Therefore, when the resist layer is developed, an extra portion is removed from the boundary zone. This reduces the uniformity of the line width and thickness of the resist layer. Therefore, if a semiconductor layer 101 is etched using the resist layer as a mask as shown in FIG. 11, the uniformity of the line width 111 (the distance from a source region 112 to a drain region 113) of the semiconductor layer 101 is reduced and therefore an etching margin is small. This causes a problem in that a semiconductor element has low dielectric strength.
Furthermore, the semiconductor layer 101 has thin boundary zones K disposed between the transflective region and the light-transmissive region; hence, a portion X that should not be heavily doped with any impurity is heavily doped with an impurity. Therefore, even if an LDD structure is formed, there is a problem in that the LDD structure malfunctions because the portion X heavily doped with the impurity is in contact with a channel region 114.